Many types of magnetic sensor apparatuses and non-contact-type electric current sensor apparatuses utilizing magnetic sensor apparatuses have been long developed since such apparatuses are useful in industry. However, their application fields have been limited and the market scale have been thus limited. Consequently, development of such apparatuses in terms of cost reduction have not been fully achieved yet.
However, emission control originating from the need for solving environmental problems has accelerated development of electric automobiles and solar-electric power generation. Since a direct current of several kilowatts to tens of kilowatts is dealt with in an electric car or solar-electric power generation, a non-contact current sensor apparatus is required for measuring a direct current of tens to hundreds of amperes. The demand for such current sensor apparatuses is extremely high. It is therefore difficult to increase the popularity of electric automobiles and solar-electric power generation unless the current sensor apparatuses not only exhibit excellent properties but also are extremely low-priced. In addition, reliability is required for a period of time as long as 10 years or more for a current sensor apparatus used in a harsh environment as in an electric car. As thus described, it has been requested in society to provide current sensor apparatuses that are inexpensive and have excellent properties and long-term reliability.
For non-contact measurement of an electric current, an alternating current component is easily measured through the use of the principle of a transformer. However, it is impossible to measure a direct current component through this method. Therefore, a method is taken to measure a magnetic field where a current is generated through a magnetic sensor for measuring a direct current component. In general, such a current sensor apparatus has a configuration including a magnetic yoke interlinking a current to be measured and having a gap in which a magnetic sensor element of a magnetic sensor apparatus is placed. A Hall element is widely used as such a magnetic sensor element incorporated in such a current sensor apparatus. A magnetoresistive (MR) element and a fluxgate element are used in some applications, too.
In applications such as an electric car or solar-electric power generation mentioned above, a current to be measured is 10 to 500 amperes. Therefore, a Hall element or a giant magnetoresistive (GMR) element suitable for measuring a high magnetic field is mainly used as a magnetic sensor element.
Not only for a current sensor apparatus incorporating a Hall element or a GMR element but also for a current sensor apparatus in general, a technique has been known for improving linearity and the dependence of output on temperature. That is, as disclosed in Published Unexamined Japanese Patent Application Sho 62-22088 (1987), for example, based on an output of a magnetic sensor apparatus, a magnetic field is generated in the direction opposite to a magnetic field to be measured that is produced by a current to be measured. Negative feedback of the output of the magnetic sensor apparatus is thereby achieved, such that the apparatus operates in the state where the magnetic field in the magnetic yoke is nearly zero, that is, in the state where the field applied to the apparatus is nearly zero. This technique is hereinafter called a negative feedback method.
For a current sensor apparatus, as disclosed in Published Examined Japanese Patent Application Sho 63-57741 (1988), for example, a technique has been known for improving measurement accuracy. That is, a specific alternating magnetic field is superposed on a magnetic field to be measured that is produced by a current to be measured. Control is performed to constantly maintain an output of the magnetic sensor apparatus responsive to the alternating magnetic field. This technique is hereinafter called an alternating current superposing method.
Various types of magnetic sensor elements have been known, such as a Hall element, an MR element, a GMR element and a fluxgate element. Each of theses elements has its own suitable measurement range of magnetic fields. Therefore, it has been required in prior art to choose a magnetic sensor element in accordance with the magnitude of a magnetic field to be measured. However, each element has its own properties such as output magnitude, linearity, and dependence on temperature. Consequently, desired accuracy is not always achieved even though a magnetic sensor element that provides a measurement range suitable for magnetic fields to be measured is chosen. Another problem is that, in some cases, no magnetic sensor element that provides a measurement range suitable for fields to be measured is available.
As described above, the negative feedback method may be applied for improving linearity and the dependence of output on temperature. However, the negative feedback method requires the generation of a negative feedback magnetic field in the opposite direction that is equal in magnitude to the field produced by the current to be measured. To measure a current of 100 amperes, for example, a feedback current of 1 ampere is required even though the number of turns of the coil for generating the negative feedback field is 100. As a result, the negative feedback method causes secondary problems such as an increase in coil dimensions, power loss, and heating. It is difficult in the prior art to solve these problems.
Furthermore, in the negative feedback method, the magnetic sensor element constantly operates in the state where the magnetic field is nearly zero. Therefore, if a Hall element whose output is small is used as the magnetic sensor element, the element is strongly affected by drifts of its own or the direct current amplification circuit and the accuracy is reduced.
With regard to a GMR element, although its output is large, it is impossible to determine the direction of a magnetic field to be measured (or the direction of a current to be measured in the case of a current sensor apparatus) since the magnetoresistive effect thereof is independent of the direction of the magnetic field. Therefore, in order to measure a magnetic field through a GMR element in the prior art, a bias magnetic field is applied such that the output of the magnetic sensor element monotonously changes in response to change in the field to be measured. In this case, however, if the direction of the field of the field to be measured is opposite to that of the bias field and the absolute value of the field to be measured exceeds that of the bias field, it is impossible to maintain the monotonicity of the output of magnetic sensor apparatus in response to changes in the field to be measured. Consequently, the negative feedback system may run away if the negative feedback method is applied.
The alternating current superposing method is a technique for improving accuracy, too. However, this method is applicable on condition that linearity of the magnetic sensor apparatus is ensured. Using this method only is thus not enough to improve linearity.
As described so far, it is impossible to measure a magnetic field or an electric current having a specific magnitude, or a great magnitude in particular, with accuracy, through the use of a magnetic sensor apparatus or a current sensor apparatus of prior art.
For example, the following problems have been found in the current sensor apparatus utilizing a Hall element that has been most highly developed in prior art.
(1) low sensitivity PA1 (2) inconsistent sensitivity PA1 (3) poor thermal characteristic PA1 (4) offset voltage that requires troublesome handling
In addition to the above problems, a magnetoresistive element has a problem of poor linearity.
Some methods have been developed for solving the problems of a Hall element. One of the methods is the negative feedback method described above. In this method, a reversed magnetic field proportional to an output of the magnetic sensor element is applied to the element so as to apply negative feedback, such that the output of the element is kept constant. Consistency in sensitivity, the thermal characteristic, and linearity are thereby improved.
When the negative feedback method is used, however, it is required to apply an inverse magnetic field as large as the field to be measured to the element. Consequently, when a current as high as hundreds of amperes is measured in applications such as an electric car or solar-electric power generation, a feedback current obtained is several amperes even if the number of turns of the coil for generating a negative feedback field is 100. Therefore, a current sensor apparatus embodied through this method is very large-sized and expensive.
If the magnetic sensor element has high sensitivity, it is possible that a feedback current is reduced by applying only part (such as one hundredth) of the field to be measured to the element. However, this is difficult for a Hall element with low sensitivity used as the magnetic sensor element.
A fluxgate element has been developed mainly for measurement of a small magnetic field while not many developments have been made in techniques for measuring a large current. However, with some modification a fluxgate element may be used as a magnetic detector of a current sensor apparatus for a large current since the fluxgate element has a simple configuration and high sensitivity.
Reference is now made to FIG. 13 to describe the operation principle of a fluxgate element having the simplest configuration. FIG. 13 is a plot for showing the relationship between an inductance of a coil wound around a magnetic core and a coil current. Since the core has a magnetic saturation property, the effective permeability of the core is reduced and the inductance of the coil is reduced if the coil current increases. Therefore, if bias magnetic field B is applied to the core by a magnet and the like, the magnitude of external magnetic field H.sub.o is measured as a change in inductance of the coil when external field H.sub.o is superposed on the bias field. This is the operation principle of the simplest fluxgate element. In FIG. 13 each of bias field B and external field H.sub.o is expressed in the magnitude converted to the coil current.
In this method the position of bias point B changes with factors such as the intensity of the magnetic field generated by the magnet or the positions of the magnet and the core in relation to each other. It is therefore required to maintain the inductance at a specific value when the external magnetic field is zero. However, it is extremely difficult to compensate for the instability of the inductance value due to temperature changes and other external perturbations. This method is therefore not suitable for practical applications.
If a rod-shaped magnetic core is used, an open magnetic circuit is provided, so that the effect of hysteresis is generally very small. Assuming that the hysteresis of the core is negligible, the characteristic of variations in inductance is equal when the coil current flows in the positive direction and in the negative direction since the saturation characteristic of the core is independent of the direction of coil current. For example, in FIG. 13, it is assumed that point P+ and point P. represent the coil current in the positive direction and the coil current in the negative direction, respectively, whose absolute values are equal. In the neighborhood of these points, the characteristic of variations in inductance with respect to variations in the absolute value of the coil current is equal. Therefore, an alternating current may be applied to the coil such that the core is driven into a saturation region at a peak, and the difference in the amounts of decrease in inductance may be measured when positive and negative peak values of the current are obtained. As a result, the difference thus measured is constantly zero when the external magnetic field is zero, which is always the case even when the characteristics of the core change due to temperature changes or external perturbations. In the present invention a saturation region of the magnetic core means a region where an absolute value of the magnetic field is greater than the absolute value of the magnetic field obtained when the permeability of the core is maximum.
An external magnetic field is assumed to be applied to the core. If external field H.sub.o is applied in the positive direction of the current, as shown in FIG. 13, the inductance value decreases at the positive peak of the current (point Q+ in FIG. 13, for example) and the inductance value increases at the negative peak of the current (point Q- in FIG. 13, for example). Therefore, the difference between the values is other than zero. Since the difference in inductance depends on the external magnetic field, the external field is obtained by measuring the difference in inductance.
The method thus described is called a large amplitude excitation method in the present invention, that is, to apply an alternating current to the coil such that the core is driven into a saturation region at a peak, and to measure the difference in the amounts of decrease in inductance when positive and negative peak values of the current are obtained.
Magnetic sensor apparatuses that utilize such a large amplitude excitation method are disclosed in Published Examined Japanese Patent Application Sho 62-55111 (1987), Published Examined Japanese Patent Application Sho 63-52712 (1988), and Published Unexamined Japanese Patent Application Hei 9-61506 (1997), for example. In Published Examined Japanese Utility Model Application Hei 7-23751 (1995), a technique is disclosed to achieve measurement similar to the large amplitude excitation method through the use of two bias magnets.
The large amplitude excitation method is an excellent method since the effects of temperature changes and external perturbations are eliminated. However, it is not so easy to apply an alternating current enough to drive the core into saturation. Accordingly, in prior art the large amplitude excitation method is limited to a magnetic sensor apparatus for detecting a small magnetic field through the use of an amorphous magnetic core and the like having a small saturation field.
For non-contact measurement of a direct current, a method is generally taken to detect a magnetic field generated by a current through the use of a magnetic sensor element. In this method, for example, a magnetic yoke having a gap is provided around a current path, and a magnetic sensor element is placed in the gap. The magnetic field in the gap is measured by the sensor element. Intensity H of the field in the gap is I/g where the current value is I and the gap length is g.
It is assumed that the negative feedback method is applied to a current sensor apparatus in which a fluxgate element made up of a single coil wound around a single magnetic core is used as a magnetic sensor element. Examples in which the negative feedback method is applied to a magnetic sensor apparatus incorporating a fluxgate element are disclosed in Published Unexamined Japanese Patent Application Sho 60-185179 (1985) and Published Unexamined Japanese Patent Application Hei 9-257835 (1997).
In the case where the negative feedback method is applied to the current sensor apparatus incorporating a fluxgate element, a magnetic field generated by the current to be measured is applied to the coil, and the magnetic field generated by the coil through a negative feedback current exactly cancels the applied magnetic field. Therefore, in order to increase the range of current to be measured, it is required to increase the negative feedback current or to reduce the applied field by increasing length g of the gap of the magnetic yoke.
However, an increase in gap g requires a large magnetic yoke, which is uneconomical. An increase in negative feedback current causes an increase in power consumption and leads to unfavorable phenomena such as heating of the coil.
In order to make the field applied to the magnetic sensor element smaller than the field to be measured, a method may be taken to shunt the magnetic flux so that only part of it passes through the magnetic sensor element. However, it is difficult in this method to precisely determine the shunt ratio.
As described so far, in order to increase the measurement range and to measure a large magnetic field or current by the magnetic sensor apparatus or the current sensor apparatus of prior art, it is required to increase the gap of the magnetic yoke or to increase the negative feedback current. In any case the above-mentioned problems are found and it is difficult to achieve the object.